Lead-free 6000 series aluminum alloy

ABSTRACT

A process for making an essentially lead-free screw machine stock alloy, comprising the steps of providing a cast aluminum ingot having a composition consisting essentially of about 0.55 to 0.70 wt. % silicon, about 0.15 to 0.45 wt. % iron, about 0.30 to 0.40 wt. % copper, about 0.8 to 0.15 wt. % manganese, about 0.80 to 1.10 wt. % magnesium, about 0.08 to 0.14 wt. % chromium, nor more than about 0.25 wt. % zinc, about 0.007 to 0.07 wt. % titanium, about 0.20 to 0.8 wt. % bismuth, about 0.15 to 0.25 wt. % tin, balance aluminum and unavoidable impurities; homogenizing the alloy at a temperature ranging from about 900° to 1060° F. for a time period of at least 1 hour; cooling to room temperature; cutting the ingot into billets; heating and extruding the billets into a desired shape; and thermomechanically treating the extruded alloy shape.

This is a division, of application Ser. No. 08/518,726, filed Aug. 24,1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lead-free aluminum screw-machinestock alloy. More specifically, the invention relates to an essentiallylead-free, tin and bismuth containing aluminum alloy screw machine stockand the process of making such an alloy.

2. Description of the Related Art

Conventional aluminum alloys used for screw machine stock contain, amongother alloying elements, lead. Workers in the field add lead toconventional aluminum screw machine stock alloys because it enhances thechipping characteristics of the alloy. There has been, however, agrowing concern regarding the health hazard created by the presence oflead in many materials including the presence of lead in conventionalaluminum alloy screw machine stock. As a result, workers in the fieldhave attempted to develop an aluminum alloy for screw machine stock thatis essentially lead-free.

Use of tin in aluminum alloys employed for mechanical cuttingoperations, such as boring, drilling or lathe-cutting, has been knownfor many years. For example, U.S. Pat. No. 2,026,571 to Kempf et al.,describes a free cutting aluminum alloy which contains copper, siliconand tin. The copper content of this cutting alloy contains 3 to 12 wt. %copper, 0.5 to 2.0 wt. % silicon, and 0.005 to 0.1 wt. % tin. It alsomay contain 0.05 to 6 wt. % of one or more of the following elements:bismuth, thallium, cadmium, or lead. In order to improve the cuttingproperties of this alloy, Kempf et al suggest subjecting it to asolution heat treatment and cold drawing.

Two other patents, U.S. Pat. Nos. 2,026,575 and 2,026,576, both to Kempfet al., describe a free cutting aluminum alloy containing 4 to 12 wt. %copper, 0.01 to 2 wt. % tin, and 0.05 to 1.5 wt. % bismuth. It mentionsthat to alter the physical properties, these alloys can be subjected tothe "usual heat treatments", but this 60 year old patent fails tospecify any particular thermomechanical steps that would assist inobtaining desirable physical properties. Moreover, both of these patentsteach that the "simultaneous presence of more than one of the freemachining elements is more advantageous than that of the same totalamount of either of the elements used separately". (See Kempf et al.'076, at column 2, lines 42-45). Specifically, Kempf et al. state that"it is more advantageous to make up this 1.5 per cent by using more thanone of the elements lead, bismuth or thallium, than to add 1.5 per centof one element alone". (See Kempf et al. '076, at column 2, lines 51 etseq.). Thus, these two patents suggest that in order to obtain the bestfree machining properties from the alloy composition, more than one freemachining elements should be added to the aluminum-copper alloy.

A more current reference, U.S. Pat. No. 5,122,208 to Alabi, discloses awear-resistant and self-lubricating aluminum alloy which containsrelatively substantial additions of tin and bismuth. This alloy has atin content of 0.5 to 3 wt. % with a corresponding quantity of bismuthcontent. It has, however, a very high silicon content and a very lowcopper level which makes it unsuitable for use as a screw machine stockalloy. Tin and bismuth containing aluminum alloys are also employed inthe manufacture of sacrificial anodes, however, the compositions of theconventional aluminum alloy sacrificial anodes make them unsuitable foruse as screw machine stock.

In addition to the aluminum screw machine stock alloy being lead-free,such an alloy should also exhibit mechanical and physical propertiesequivalent to its lead-containing counterparts. Thus, a need remains foran aluminum screw machine stock alloy that is lead-free while stillmaintaining mechanical and physical properties equivalent to itslead-containing screw machines stock alloy counterparts. Accordingly, itis an object of this invention to provide such an alloy.

SUMMARY OF THE INVENTION

The present invention comprises an essentially lead-free, extruded andthen solution heat-treated aluminum screw machine stock alloy consistingessentially of about 0.40 to 0.8 wt. % silicon, not more than about 0.7wt. % iron, about 0.15 to 0.40 wt. % copper, not more than about 0.15wt. % manganese, about 0.8 to 1.2 wt. % magnesium, about 0.04 to 0.14wt. % chromium, not more than about 0.25 wt. % zinc, not more than about0.15 wt. % titanium, about 0.10 to 0.7 wt. % tin, and about 0.20 to 0.8wt. % bismuth, balance aluminum and unavoidable impurities.

The process of making such an alloy includes the steps of homogenizingthe ingot at a temperature ranging from about 900° to 1060° F. for atime period of at least 1 hour, cooling, cutting the ingot into billets,heating and extruding the billets into a desired shape, andthermomechanically treating the extruded alloy shape.

The foregoing and other objects, features, and advantages of theinvention will become more readily apparent from the following detaileddescription of preferred embodiment which proceeds with reference to thedrawings.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a lead-free aluminum screw-machinestock alloy and the process for making such alloy. More specifically,the invention relates to an essentially lead-free, tin and bismuthcontaining aluminum alloy screw machine stock and the process of makingsuch an alloy. We have found that if we replace the lead content of theconventional aluminum alloy for screw machine stock with a quantity oftin, and then subject that alloy to thermal mechanical treatment, weobtain an alloy that exhibits at least the equivalent physical andmechanical properties exhibited by the lead containing aluminum screwmachine stock alloy without encountering any significant health hazardswhich the conventional lead-containing alloys may create.

Aluminum screw machine stock is generally manufactured in the rod or barform to be used in screw machines. Aluminum alloy screw machine stockmust exhibit the best possible machinability and chip breakagecharacteristics for that particular alloy. Along with exhibiting goodmachinability and chip breakage the material must satisfy the physicaland mechanical properties required for the end use product. Thoseproperties were obtained in the past when a lead containing alloygenerally having a lead content of about 0.50 wt. % and designated bythe Aluminum Association as AA 6262 alloy was utilized for making screwmachine stock. There are, however, concerns that operators who aresubjected to prolonged exposure to lead-containing screw machine stock,such as AA 6262, may experience harmful health effects. These concernshave created a need for a lead-free screw machine stock alloy to replaceits lead-containing predecessor. The mechanical, physical andcomparative characteristics of the lead-free aluminum screw machinestock alloy should perform in at least an equivalent manner to theconventional lead containing 6262 aluminum screw machine stock alloy.

The aluminum alloy of the present invention provides a suitablereplacement alloy for the conventional 6262 alloy without the possibleproblems created by lead that is contained in the conventional alloy.Also the alloy of the present invention exhibits a degree ofmachinability in chip breakage characteristics that were expected forthe lead containing aluminum alloy screw machine stock withoutsacrificing any of the physical, mechanical and comparativecharacteristics of the alloy. The physical properties of the alloy aredependent upon a chemical composition that is closely controlled withinspecific limits as set forth below and upon carefully controlled andsequenced process steps. If the composition limits or process parametersstray from the limits set forth below, the desired combination of beinglead-free and important machinability properties will not be achieved.

Our invention alloy consists essentially of about 0.40 to 0.8 wt. %silicon, not more than about 0.7 wt. % iron, about 0.15 to 0.40 wt. %copper, not more than about 0.15 wt. % manganese, about 0.8 to 1.2 wt. %magnesium, about 0.04 to 0.14 wt. % chromium, not more than about 0.25wt. % zinc, not more than about 0.15 wt. % titanium, about 0.10 to 0.7wt. % tin, and about 0.20 to 0.8 wt. % bismuth, balance aluminum andunavoidable impurities. Our preferred alloy consists essentially ofabout 0.55 to 0.7 wt. % silicon, not more than about 0.45 wt. % iron,about 0.30 to 0.4 wt. % copper, not more than about 0.15 wt. %manganese, about 0.8 to 1.1 wt. % magnesium, about 0.08 to 0.14 wt. %chromium, not more than about 0.25 wt. % zinc, not more than about 0.07wt. % titanium, about 0.15 to 0.25 wt. % tin, and about 0.50 to 0.74 wt.% bismuth, balance aluminum and unavoidable impurities.

We have found that if the alloys contains less than 0.10 wt. % tin, itdoes not chip well. If, however, the alloy contains more than 0.7 wt. %tin or more than 0.8 wt. % bismuth there is little, if any, beneficialeffect. In addition, at higher levels of tin, the chipping and tool lifeis diminished.

In addition, we have found that by further narrowing the bismuth and tinranges we can obtain additional benefits. Thus, our most preferred alloyincludes bismuth ranging from about 0.50 to 0.74 wt. % and tin rangingfrom about 0.10 to 0.7 wt. % and even more preferably from about 0.15 to0.25 wt. %. We have found that by further limiting the range of bismuthand tin we obtain optimum chipping and tool life for the alloy.

Initially, we cast the alloy into ingots and homogenize the ingots at atemperature ranging from about 1000° to 1170° F. for at least 1 hour butgenerally not more than 24 hours followed either by fan or air cooling.Preferably, we soak the ingot at about 1020° F. for about 4 hours andthen cool to room temperature. Next, we cut the ingots into shorterbillets, heat them to a temperature ranging from about 600° to 720° F.and then extrude the billets into a desired shape, generally a rod orbar form.

We then thermomechanically treat the extruded alloy shape to obtain thedesired mechanical and physical properties. For example, to obtain themechanical and physical properties of a T8 temper, we solution heattreat at a temperature ranging from about 930° to 1030° F., preferablyat about 1000° F., for a time period ranging from about 0.5 to 2 hours,rapidly quench the heat-treated shape to room temperature, cold work theshape, and artificial age the cold worked shape at a temperature rangingfrom about 300° to 380° F. for about 4 to 12 hours.

To obtain a T4 temper, we cold work the shape, solution heat treat theextruded alloy shape at a temperature ranging from about 930° to 1030°F. for a time period ranging from about 0.5 to 2 hours, rapidly quenchthe heat-treated shape to room temperature, then straighten using anyknown straightening operation such as stress relieved stretching ofabout 1 to 3 % and naturally age the cold worked shape. To impart a T6or T651 temper we further artificially age the T4 or T45 1 straightenedshape. The artificial age cycle would be carried out in the range fromabout 300° to 380° F. for about 4 to 12 hours.

To obtain a T4 or T4511 temper, we solution heat treat at a temperatureranging from about 930° to 1030° F. for a time period ranging from about0.5 to 2 hours, rapidly quench the heat-treated shape to roomtemperature, the shape can then be straightened by using knownstraightening operations such as stress relieved stretching of about 1to 3%, and allow the shape to naturally age. To impart a T6 T6511 temperwe further artificially age the T4 or T4511 shape. The artificial agecycle would be carried out in the range from about 300° to 380° F. forabout 4 to 12 hours.

To obtain the properties of a T6 of T6511 temper, prior to extrusion, weheat the billets to a temperature ranging from about 950° to 1050° F.and then extrude them to a near desired size in rod or bar form.Subsequent to the extrusion process, we rapidly quench the alloy to roomtemperature to minimize uncontrolled precipitation of the alloyingconstituents. The rod or bar is then straightened using any knownstraightening operation such as stress relieved stretching of about 1 to3 %. To further improve its physical and mechanical properties, wefurther heat treat the alloy by precipitation ar artificial agehardening. We generally accomplish this heat treatment step at atemperature ranging from about 300° to 380° F. for a time period fromabout 4 to 12 hours.

To obtain a T9 temper, we subject the extruded stock to a solution heattreatment at a temperature ranging from about 930° to 1030° F. for atime period ranging from about 0.5 to 2 hours, rapidly quench theheat-treated stock to room temperature, artificially age the stock at atemperature ranging from about 300° to 380° F. for a time period rangingfrom about 4 to 12 hours, and then we cold work the stock followed byany known straightening operation such as roll straightening.

EXAMPLE

To demonstrate the present invention, I first prepared alloys of thecompositions shown in Table 1 as cast ingots, which were thenhomogenized at 1040° F for 4 hours, cooled to room temperature, cut tobillet, reheated to 600° F, extruded into 1.188" diameter stock,solution heat treated at 1000° F for 30 minutes then rapid quenchedusing water and and aged at 350F for 8 hours (T8 temper).

                                      TABLE 1                                     __________________________________________________________________________    CHEMICAL COMPOSITIONS OF ALLOYS                                               Alloy No.                                                                          Si Fe Cu Mn Mg Cr Zn Pb (*)                                                                            Bi  Sn                                          __________________________________________________________________________    1 (**)                                                                             0.608                                                                            0.296                                                                            0.268                                                                            0.11                                                                             0.98                                                                             0.10                                                                             0.016                                                                            0.609                                                                             0.62                                                                              --                                          2    0.64                                                                             0.356                                                                            0.405                                                                            0.126                                                                            1.028                                                                            0.12                                                                             0.003                                                                            --  --  0.20                                        3    0.64                                                                             0.365                                                                            0.333                                                                            0.108                                                                            1.01                                                                             0.105                                                                            0.005                                                                            0.018                                                                             0.316                                                                             0.20                                        4    0.585                                                                            0.338                                                                            0.307                                                                            0.10                                                                             0.997                                                                            0.101                                                                            0.007                                                                            0.017                                                                             0.587                                                                             0.20                                        5    0.591                                                                            0.291                                                                            0.282                                                                            0.09                                                                             0.968                                                                            0.094                                                                            0.007                                                                            0.036                                                                             0.002                                                                             0.38                                        6    0.625                                                                            0.277                                                                            0.292                                                                            0.103                                                                            0.994                                                                            0.107                                                                            0.005                                                                            0.037                                                                             0.446                                                                             0.38                                        __________________________________________________________________________     (*) Trace element in primary material charged to make alloy                   (**) This alloy represents typical AA6262.                               

The mechanical properties for each of the alloys were tested and theresults are in Table 2.

                  TABLE 2                                                         ______________________________________                                        MECHANICAL PROPERTIES OF                                                      T8 TEMPER MATERIAL (AVERAGED)                                                          Ultimate Tensile                                                                           Yield Tensile                                                                             Elongation                                  Alloy No.                                                                              Strength ksi Strength ksi                                                                              % in 2-in.                                  ______________________________________                                        1        53.4         52.0        13.5                                        2        55.3         54.0        13.0                                        3        54.4         52.7        13.0                                        4        52.0         50.5        13.2                                        5        53.8         52.4        12.0                                        6        51.2         50.0        12.5                                        ______________________________________                                    

The data show that the six alloys have similar mechanical properties.The distribution of the data is typical for a 6262.T8 product.

Table 3 gives the results of the machine testing performed on eachalloy.

                  TABLE 3                                                         ______________________________________                                        MACHINABILITY DATA                                                                    Tool Life - Hours                                                                           Surface Finish                                                                             Chip Size                                  Alloy No.                                                                             to 0.005" Growth                                                                            Roughness Ave.                                                                             (Note 1)                                   ______________________________________                                        1       2.5           23                                                      2       4.0           24                                                      3       6.0           26                                                      4       5.5           37                                                      5       5.0           21                                                      6       2.5           24                                                      ______________________________________                                    

(Note 1) Chip classification is difficult to quantify so the chips arerated by comparing one to another. The chips from Alloy No. 1 were wellbroken. Ths chips from Alloys No. 2 and 4 are slightly larger than AlloyNo. 1 chips but are very similar. The chips from Alloys No. 3, 5 and 6are larger in size than Alloy No. 1 and not as compact.

All six alloys were tested for anodize performance. Table 4 shows theresults of that work.

                  TABLE 4                                                         ______________________________________                                        ANODIZE PERFORMANCE                                                                                         Bright Dip, Sulfuric                            Alloy No.                                                                             Hardcoat  Sulfuric Acid                                                                             Acid and Dye                                    ______________________________________                                        1       Good      Good        Good                                            2       Good      Good        Good                                            3       Good      Good        Good                                            4       Good      Good        Good                                            5       Good      Good        Good                                            6       Good      Good        Good                                            ______________________________________                                    

These data show that the alloys have equivalent anodize qualities andmetallurgical structure anomalies were not seen.

Having illustrated and described the principles of my invention in apreferred embodiment thereof, it should be readily apparent to thoseskilled in the art that the invention can be modified in arrangement anddetail without departing from such principles. I claim all modificationscoming within the spirit and scope of the accompanying claims.

We claim:
 1. A process for making an essentially lead-free screw machinestock alloy, comprising the steps of:(a) providing a cast aluminum ingothaving a composition consisting essentially of about 40 to 0.8 wt. %silicon, not more than about 0.7 wt. % iron, about 0.15 to 0.40 wt. %copper, not more than about 0.15 wt. % manganese, about 0.8 to 1.2 wt. %magnesium, about 0.04 to 0.14 wt. % chromium, not more than about 0.25wt. % zinc, not more than about 0.15 wt. % titanium, about 0.10 to 0.7wt. % tin, and about 0.20 to 0.8 wt. % bismuth, balance aluminum andunavoidable impurities; (b) homogenizing the ingot at a temperatureranging from about 900° to 1060° F. for a time period of at least 1hour; (c) cooling; (d) cutting the ingot into billets; (e) heating andextruding the billets into a desired shape; and (f) thermomechanicallytreating the extruded alloy shape.
 2. The process of claim 1, whereinthe thermomechanical treatment step comprises:(i) solution heat treatingat a temperature ranging from about 930° to 1030° F. for a time periodranging from about 0.5 to 2 hours; (ii) rapid quenching of theheat-treated shape to room temperature; (iii) cold working the quenchedshape; and (iv) artificial aging the cold worked shape to impart a T8temper.
 3. The process of claim 1, wherein the thermomechanicaltreatment step comprises:(i) cold working the shape; (ii) solution heattreating the cold worked shape at a temperature ranging from about 930°to 1030° F. for about 0.5 to 2.0 hours; (iii) rapid quenching of theheat-treated shape to room temperature; and (iv) natural aging thequenched, heat-treated shape to impart a T4 temper.
 4. The process ofclaim 3, further comprising stretching prior to natural aging to imparta T451 temper.
 5. The process of claim 3, further comprising artificialaging to impart a T6 temper.
 6. The process of claim 5, wherein theartificial aging step comprises heating from about 300° to 380° F. forabout 4 to 12 hours.
 7. The process of claim 4, further comprisingartificial aging to impart a T651 temper.
 8. The process of claim 1,wherein the thermomechanical step comprises:(i) solution heat treatingat a temperature ranging from about 930° to 1030° F. for a time periodranging from about 0.5 to 2 hours; (ii) rapid quenching of theheat-treated shape to room temperature; (iii) naturally aging to imparta T4 temper.
 9. The process of claim 7 wherein the artificial aging stepcomprises heating from about 300° to 380° F. for about 4 to 12 hours.10. The process of claim 8 further comprising straightening prior tonatural aging to impart a T4511 temper.
 11. The process of claim 1wherein the thermomechanical step comprises:(i) solution heat treatingat a temperature ranging from about 930° to 1030° F. for a time periodranging from about 0.5 to 2 hours; (ii) rapid quenching of theheat-treated shape to room temperature; (iii) artificial aging; (iv)cold working; and (v) straightening to impart a T9 temper.
 12. Theprocess of claim 1 further comprising artificial aging about to impart aT6511 temper.
 13. The process of claim 1 further comprising artificialaging about to impart a T6 temper.
 14. The process of claim 12 whereinthe artificial aging step comprises heating from about 300° to 380° F.for about 4 to 12 hours to impart a T6511 temper.
 15. The process ofclaim 13 wherein the artificial aging step comprises heating from about300° to 380° F. for about 4 to 12 hours to impart a T6 temper.